In analog front-end circuits, a very small analog signal obtained by detecting a temperature, for example, is amplified and AD-converted into data. In this operation, an offset component, a temperature drift, low-frequency noise, etc. (hereinafter referred to as “an offset component etc.”) occurring in an instrumentation preamplifier having a high amplification factor and an AD converter have great influences on the performance of the entire circuit.
In view of the above, for example, an analog front-end circuit 20A shown in FIG. 3 is employed which is equipped with a configuration for reducing such offset component etc. (refer to Chopping on the AD7190, AD7192, AD7193, AD7194, and AD7195,” Mary McCarthy, AN-1131, APPLICATION NOTE, page 1 of 4, ANALOG DEVICES). The analog front-end circuit 20A is equipped with switches SW11 and SW12 each of which switches between input voltages VINP and VINN that are input from a sensor 1, a differential instrumentation preamplifier 21A which performs differential amplification on the difference between the voltages VINP and VINN that are input via the switches SW11 and SW12, an AD converter 22 for converting the difference between differential output signals VOUTP and VOUTN of the differential instrumentation preamplifier 21A into a digital signal, a switch circuit 23 which passes the output data of the AD converter 22 as it is or after inverting its polarity, and a data calculator 24 for calculating average data of two data that are output from the switch circuit 23 successively.
The differential instrumentation preamplifier 21A is composed of operational amplifiers OP11 and OP12 and gain-setting resistors R11, R12, and R13 (R12=R13), and outputs the voltages VOUTP and VOUTN. The switch circuit 23 is composed of a switch SW13 and a data inverter 23a. 
A chopping signal CHOP is a signal that “0” and “1” occur repeatedly and alternately at a prescribed cycle. Each of the switches SW11, SW12, and SW13 selects the input terminals “0” and “1” when the chopping signal CHOP is “0” and “1,” respectively.
In FIG. 3, symbols Vos11, Vos12, and Vos13 denote an offset component etc. occurring on the non-inverting input side of the operational amplifier OP11, an offset component etc. occurring on the non-inverting input side of the operational amplifier OP12, and an offset component etc. occurring on the data input side of the AD converter 22, respectively.
In the analog front-end circuit 20A, an output voltage VOCHOP=0 of the switch circuit 23 when the chopping signal CHOP is “0” is given by
                              VO                      CHOP            =            0                          =                                            (                              VINP                -                VINN                            )                        ×                          (                              1                +                                  2                  ×                                                            R                      ⁢                                                                                          ⁢                      12                                                              R                      ⁢                                                                                          ⁢                      11                                                                                  )                                +                                    (                                                Vos                  ⁢                                                                          ⁢                  11                                -                                  Vos                  ⁢                                                                          ⁢                  12                                            )                        ×                          (                              1                +                                  2                  ×                                                            R                      ⁢                                                                                          ⁢                      12                                                              R                      ⁢                                                                                          ⁢                      11                                                                                  )                                +                      Vos            ⁢                                                  ⁢            13.                                              (        1        )            
On the other hand, an output voltage VOCHOP=1 of the switch circuit 23 when the chopping signal CHOP is “1” is given by
                              VO                      CHOP            =            1                          =                                            {                                                                    -                                          (                                              VINP                        -                        VINN                                            )                                                        ×                                      (                                          1                      +                                              2                        ×                                                                              R                            ⁢                                                                                                                  ⁢                            12                                                                                R                            ⁢                                                                                                                  ⁢                            11                                                                                                                )                                                  +                                                      (                                                                  Vos                        ⁢                                                                                                  ⁢                        11                                            -                                              Vos                        ⁢                                                                                                  ⁢                        12                                                              )                                    ×                                      (                                          1                      +                                              2                        ×                                                                              R                            ⁢                                                                                                                  ⁢                            12                                                                                R                            ⁢                                                                                                                  ⁢                            11                                                                                                                )                                                  +                                  Vos                  ⁢                                                                          ⁢                  13                                            }                        ×                          (                              -                1                            )                                =                                                    (                                  VINP                  -                  VINN                                )                            ×                              (                                  1                  +                                      2                    ×                                                                  R                        ⁢                                                                                                  ⁢                        12                                                                    R                        ⁢                                                                                                  ⁢                        11                                                                                            )                                      -                                          (                                                      Vos                    ⁢                                                                                  ⁢                    11                                    -                                      Vos                    ⁢                                                                                  ⁢                    12                                                  )                            ×                              (                                  1                  +                                      2                    ×                                                                  R                        ⁢                                                                                                  ⁢                        12                                                                    R                        ⁢                                                                                                  ⁢                        11                                                                                            )                                      -                          Vos              ⁢                                                          ⁢              13.                                                          (        2        )            
Thus, an output OUT of the data calculator 24, that is, the average of two successive output voltages VOCHOP=0 and VOCHOP=1 of the switch circuit 23, is given by
                    OUT        =                                                            VO                                  CHOP                  =                  0                                            +                              VO                                  CHOP                  =                  1                                                      2                    =                                    (                              VINP                -                VINN                            )                        ×                                          (                                  1                  +                                      2                    ×                                                                  R                        ⁢                                                                                                  ⁢                        12                                                                    R                        ⁢                                                                                                  ⁢                        11                                                                                            )                            .                                                          (        3        )            In Equation (3), the offset components etc. Vos11, Vos12, and Vos13 are eliminated.
However, in the analog front-end circuit 20A described above with reference to FIG. 3 which employs the differential instrumentation preamplifier 21A, output voltages VOUTP and VOUTN of the differential instrumentation preamplifier 21A vary necessarily in opposite directions (differentially). Thus, where the bias points for input signals VINP and VINN are set at low potentials that are close to GND, the output voltages VOUTP and VOUTN are saturated immediately.
FIG. 5C shows example input voltages VINP and VINN and output voltages VOUTP and VOUTN of the differential instrumentation preamplifier 21A of the analog front-end circuit 20A shown in FIG. 3 in which the input voltages VINP and VINN are 2 V and 1 V, respectively. When the chopping signal CHOP is “0,” the input voltages VINP and VINN pass the switches SW11 and SW12 as they are, respectively, and hence the output voltages VOUTP and VOUTN and the amplification factor GAIN are given by
                    VOUTP        =                  VINP          +                                                    R                ⁢                                                                  ⁢                12                                            R                ⁢                                                                  ⁢                11                                      ×                          (                              VINP                -                VINN                            )                                                          (        4        )                                VOUTN        =                  VINN          +                                                    R                ⁢                                                                  ⁢                12                                            R                ⁢                                                                  ⁢                11                                      ×                          (                              VINN                -                VINP                            )                                                          (        5        )                                GAIN        =                                            VOUTP              -              VOUTN                                      VINP              -              VINN                                =                      1            +                          2              ×                                                R                  ⁢                                                                          ⁢                  12                                                  R                  ⁢                                                                          ⁢                  11                                                                                        (        6        )            
Where the amplification factor GAIN is 10, R12/R11 is calculated to be 4.5 from Equation (6). Thus, the output voltages VOUTP and VOUTN are calculated asVOUTP=2+4.5×1=6.5(V)VOUTN=1+4.5×(−1)=−3.5(V).
On the other hand, when the chopping signal CHOP is “1,” the input voltages VINP and VINN are inverted by the switches SW11 and SW12 and hence the output voltages VOUTP and VOUTN are given by
                    VOUTP        =                  VINN          +                                                    R                ⁢                                                                  ⁢                12                                            R                ⁢                                                                  ⁢                11                                      ×                          (                              VINN                -                VINP                            )                                                          (        7        )                                VOUTN        =                  VINP          +                                                    R                ⁢                                                                  ⁢                12                                            R                ⁢                                                                  ⁢                11                                      ×                                          (                                  VINP                  -                  VINN                                )                            .                                                          (        8        )            
Thus,VOUTP=1+4.5×(−1)=−3.5(V)VOUTN=2+4.5×1=6.5(V).
As described above, in the analog front-end circuit 20A shown in FIG. 3, theoretically, the output voltages VOUTP and VOUTN can become negative. As a result, where a single power source is employed for the operational amplifiers OP11 and OP12, the negative side is saturated to 0 V. Thus, to handle a single-ended signal in which only a signal VINP that is output from the sensor 1 varies on the positive side and a voltage VINN that is output from the sensor 1 is fixed, it is necessary to set high bias points for the voltages VINP and VINN in advance, which results in a problem that the signal range of the output voltage VOUTP is narrowed.
One measure for widening the signal range is an analog front-end circuit 20B shown in FIG. 4 which employs a non-inverting instrumentation preamplifier 21B which uses a single operational amplifier OP11. FIG. 5D shows example input voltages VINP and VINN and output voltages VOUTP and VOUTN of the differential instrumentation preamplifier 21B of the analog front-end circuit 20B shown in FIG. 4.
When the chopping signal CHOP is “0”.
                    VOUTP        =                  VINP          +                                                    R                ⁢                                                                  ⁢                12                                            R                ⁢                                                                  ⁢                11                                      ×                          (                              VINP                -                VINN                            )                                                          (        9        )                                VOUTN        =        VINN                            (        10        )                                GAIN        =                                            VOUTP              -              VOUTN                                      VINP              -              VINN                                =                      1            +                                          R                ⁢                                                                  ⁢                12                                            R                ⁢                                                                  ⁢                11                                                                        (        11        )            
Where the amplification factor GAIN is 10, R12/R11 is calculated to be 9 from Equation (11). Thus, the output voltages VOUTP and VOUTN are calculated asVOUTP=2+9×1=11(V)VOUTN=1(V).
On the other hand, when the chopping signal CHOP is “1.” the output voltages VOUTP and VOUTN are given by
                    VOUTP        =                  VINN          +                                                    R                ⁢                                                                  ⁢                12                                            R                ⁢                                                                  ⁢                11                                      ×                          (                              VINN                -                VINP                            )                                                          (        12        )                                VOUTN        =                  VINP          .                                    (        13        )            
Thus,VOUTP=1+9×(−1)=−8(V)VOUTN=2(V).
In this case, when the chopping signal CHOP is “0,” neither of the output voltages VOUTP and VOUTN is saturated and hence a wide signal range can be obtained. However, when the chopping signal CHOP is “1,” the output voltage VOUTP becomes negative, which raises the same problem as in the analog front-end circuit 20A shown in FIG. 3.
One object of the invention is to solve the above problems and thereby provide an analog front-end circuit capable of eliminating offset components etc. by a chopper operation and, in addition, preventing saturation of output voltages of an instrumentation preamplifier in a case of handling a single-ended signal having a wide signal range.